Electrosurgical dispersive electrodes are now known, and such electrodes are now utilized to provide a return current path to an electrosurgical generator from a patient receiving therapeutic signals from the generator through an active electrode in contact with the patient.
Electrosurgical dispersive electrodes now known and/or utilized may be either for single-use (i.e., disposable) or multiple-use (i.e., re-usable), and may be typified as either resistive-contact or capacitive-contact electrodes.
Resistive-contact electrodes now known and/or utilized employ a bare metallic electrode in contact with the skin or an electrolytically conductive gel between and in engagement with a metallic electrode and the skin surface. Because of the characteristics of the resulting electrical boundary conditions, the current is not evenly distributed over the surface of the electrode, with the current density being much higher at the edges than at the center of the electrode. This results in a hot perimeter, and a patient is more likely to receive a burn at the edge of the electrode because of the uneven current distribution. Resistive-contact electrodes have an advantage, however, in that the electrolytic gel provides a low impedance contact. Moreover, the electrolytic gel at room temperature cools the skin slightly when the electrode is applied and this results in lessening of the skin-temperature rise.
A resistive-contact type of electrode used as an electro-surgical dispersive electrode is shown, by way of example, in U.S. Pat. Nos. 4,088,133 and 3,848,600.
Presently known and/or utilized capacitive-contact electrode, on the other hand, have a thin dielectric film between the metallic electrode and the skin. The skin forms one "plate" of a capacitor, while the metallic electrode forms the other plate to thus establish capacitive contact. The electrical boundary conditions for capacitive-contact electrodes are such that, if ideal capacitive contact is made, the current is necessarily uniformly distributed over the electrode surface, and there is no hot perimeter. A further advantage of capacitive electrodes is that they are more convenient to store and apply than resistive-contact electrodes.
Known capacitive-contact electrodes, however, have disadvantages, including: lack of a conducting gel which cools (thus the final temperature may be higher than with gelled electrodes); requirement of use of the skin as a "plate" (the skin also has dielectric properties and is an unpredictable participant in the capacitive coupling process); difficulty of uniform capacitive electrode-skin separation (uniform contact is not easily achievable in practice and buckling of the electrode causes hot spots); and creation of slightly high impedance (the capacitive electrode exhibits a slightly higher impedance than gel-contact electrodes).
Capacitive-contact electrodes used as electrosurgical dispersive electrodes are shown, by way of example, in U.S. Pat. Nos. 4,200,104, 4,188,927, and 4,166,465.